You don't HAVE to be faithful to your wife.
You don't HAVE to give up your seat to an elderly woman with a respirator.
You don't HAVE to help a lost child find his mom.
You don't HAVE to tell someone they just dropped their wallet.
You don't HAVE to take responsibility for your actions.
You don't HAVE to tell the truth.
You don't HAVE to do any number of things that the social contract expects of you, even though it cannot demand them.